This application claims benefits of Japanese Application No. 2002-36267 filed in Japan on Feb. 14, 2002, the contents of which are incorporated by this reference.
The present invention relates generally to a zoom lens and an image pickup apparatus using the same, and more particularly to a high-zoom-ratio zoom lens that is well fit for cameras, especially video cameras and digital still cameras.
Home-video cameras are now increasingly diminished, and the market is fairly mature. About 10:1 power zoom lenses are mainly used for phototaking lenses. Recently, on the other hand, attention has been focused on digital still cameras (electronic still cameras) as the coming generation of cameras that are taking the place of silver-halide 35 mm-film (usually called Leica size) cameras. Designed to provide stills, the digital still cameras must satisfy higher image quality levels than do home-video cameras, and so even general user-oriented digital cameras rely primarily on image pickup devices having 2,000,000 pixels. It is thus difficult to design compact, high-zoom-ratio zoom lenses unlike the case of home-video cameras, and so about 3:1 power zoom lenses are mainly used. The market for digital still cameras having such a 3:1 power zoom lens are coming of age, and so the advent of higher-zoom-ratio zoon lenses is desired. However, most of compact, high-zoom-ratio zoom lenses currently available for electronic image pickup apparatus are oriented for home videos having a small number of pixels. When such zoom lenses are used with digital cameras, it is impossible to allow image pickup devices to take full advantage of their own resolving power.
For instance, if an image pickup device having about 2,000,000 pixels is used with a zoom lens of the construction exemplified in the inventive examples given later, that image pickup device may take full advantage of its own resolving power. However, the spectral sensitivity properties of that image pickup device including a color filter, unlike those of silver-halide color film, are not faithful for the specific features of the human eyes (which have higher sensitivity to wavelengths of about 400 to 430 nm than required, low sensitivity to wavelengths of about 600 nm, and high sensitivity to wavelengths of 700 nm or higher not originally in existence). For this reason, those spectral sensitivity properties are largely affected by chromatic aberrations of the lens in near-ultraviolet and near-infrared ranges, resulting in chromatic blurring and, hence, considerable damage to image quality. Color reproducibility to plants and many other subjects having strong reflection spectra in the infrared range in general, too, is considerably damaged.
The chromatic blurring of a phototaken image is particularly noticeable at a subject site having a large brightness difference. Referring to the boundary between a high brightness site and a low brightness site, this is explained as follow. The high brightness site is whitened beyond the latitude of an image pickup device due to over-exposure whereas light of wavelengths of particularly about 400 to 430 nm on a high brightness side aberrates as chromatic aberrations on a low brightness side. In view of light quantity, on the other hand, the aberrating light comes within the range of the latitude of the image pickup device. In addition, the relative sensitivity of this wavelength range becomes much higher as compared with silver-halide color film. Consequently, a portion of the low brightness side near to that boundary is strikingly colored in purple. When it comes to an optical system having a high zoom ratio and a largely asymmetric power profile wherein the stronger the power of each lens element becomes due to compactness, the more noticeable chromatic aberrations due to secondary spectra becomes, the wavelengths of 400 to 430 nm that are main chromatic blurring components are achromatized with wavelengths in the vicinity of 550 nm having the highest sensitivity. However, chromatic aberrations in the range of 500 to 600 nm having relatively high sensitivity start to occur gradually, and so the possibility of improvements in image quality due to an increase in the number of pixels of the image pickup device is spoiled by the image-formation capability of the zoom lens. Accordingly, to eliminate a chromatic blurring problem while sharpness is maintained, there is no option but to make correction for secondary spectra or removing light components of 400 to 430 nm wavelengths that are main chromatic blurring components by means of filters, etc. without detrimental to color reproduction.
In view of such situations that the number of pixels of an image pickup device continues to increase whereas the pixel pitch becomes increasingly narrow, the object of the present invention is to provide a digital still camera which is compatible with the resolving power of a coming image pickup device having as many pixels as about 3,000,000 or greater and uses a zoom lens having a high zoom ratio yet compact, simple construction, wherein chromatic aberrations of the zoom lens are reduced and the zoom lens is kept against the influences of chromatic aberrations.
According to the first aspect of the invention, this object is achieved by the provision of a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
a doublet component of said two doublet components, which component is located on an image side thereof, has a meniscus form concave on an image side thereof.
According to the second aspect of the invention, there is provided a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
a doublet component of said two doublet components, which component is located on an image side thereof, has negative refracting power.
According to the third aspect of the invention, there is provided a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
said two doublet components satisfy the following conditions (1) and (2):
0.000 less than AVE[(xcex94xcex8gF)C1p, (xcex94xcex8gF)C2p] less than 0.080xe2x80x83xe2x80x83(1)
xe2x88x920.030 less than AVE[(xcex94xcex8gF)C1n, (xcex94xcex8gF)C2n] less than 0.003xe2x80x83xe2x80x83(2)
where
(xcex94xcex8gF)C1P is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C1n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C2p is the anomalous dispersibility of a medium forming the positive lens element of the doublet component of said two doublet components, which is located on the image side of said positive lens group,
(xcex94xcex8gF)C2n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group,
AVE[(xcex94xcex8gF)C1p , (xcex94xcex8gF)C2p ] is the arithmetic mean of (xcex94xcex8gF)C1p and (xcex94xcex8gF)C2p , and
AVE[(xcex94xcex8gF)C1n , (xcex94xcex8gF)C2n ] is the arithmetic mean of (xcex94xcex8gF)C1n and (xcex94xcex8gF)C2n .
According to the fourth aspect of the invention, there is provided a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
said two doublet components satisfy at least one of the following conditions (3-1) and (3-2) and at least one of the following conditions (4-1) and (4-2):                               xe2x80x83                ⁢                  0.0000           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C1p                     less than           0.1000                                    (                  3          ⁢                      -                    ⁢          1                )                                          xe2x80x83                ⁢                  0.0000           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C2p                     less than           0.1000                                    (                  3          ⁢                      -                    ⁢          2                )                                          -          0.0300                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C1n                 less than         0.0000                            (                  4          ⁢                      -                    ⁢          1                )                                          -          0.0300                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C2n                 less than         0.0000                            (                  4          ⁢                      -                    ⁢          2                )            
where
(xcex94xcex8gF)C1p is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
( xcex94xcex8gF)C1n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C2p is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group, and
(xcex94xcex8gF)C2n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group.
Why the aforesaid arrangements are used herein, and how they work is now explained.
Basically, the zoom lens of the invention includes a lens group having negative refracting power and at least one lens group A located subsequent thereto and having positive refracting power, wherein the spacing between these lens groups changes upon zooming, and the lens group A comprises a plurality of lens components.
For instance, if the lens group A is composed of one positive lens element and one negative lens element, it is preferable for the purpose of slacking chromatic aberrations due to secondary spectra to roughly satisfy the following conditions (a) and (b) regarding the anomalous dispersibility of the media that form the respective lens elements.
0.0000 less than (xcex94xcex8gF)pxe2x80x83xe2x80x83(a)
(xcex94xcex8gF)n less than 0.0000xe2x80x83xe2x80x83(b)
Here (xcex94xcex8gF)p and (xcex94xcex8gF)n are the anomalous dispersibility of the media forming the positive lens element and the negative lens element in the lens group A, respectively.
The definition of the anomalous dispersibility xcex94xcex8gF of each medium (vitreous material) is now explained.
xcex8gF=AgF+BgFxc2x7xcexdd+xcex94xcex8gF
where
xcex8gF=(ngxe2x88x92nF)/(nFxe2x88x92nC),
xcexdd=(ndxe2x88x921)/(nFxe2x88x92nC), and
AgF and BgF are each a linear coefficient determined by two vitreous materials, glass code 511605 (NSL7 made by Ohara Co., Ltd.; xcex8gF=0.5436 and xcexdd=60.49) and glass code 620363 (PBM2 made by Ohara Co., Ltd.; xcex8gF=0.5828 and xcexdd=36.26). It is here noted that ng, nF, nC and nd are the refractive indices of the medium with respect to g-line, F-line, C-line and d-line wavelengths, respectively, and xcex8gF is the partial dispersion ratio of the medium.
That is, ⊕xcex8gF is the amount of displacement of the vitreous material in a xcex8gF direction on the basis of a straight line between glass code 511605 (NSL7 made by Ohara Co., Ltd.; xcex8gF=0.5436 and xcexdd=60.49) and glass code 620363 (PBM2 made by Ohara Co., Ltd.; xcex8gF=0.5828 and xcexdd=36.26) in a partial dispersion ratio xcex8gF vs. Abbe number xcexdd plot of the vitreous material or a numerical representation of anomalous dispersibility.
Specifically in the defining formula for xcex94xcex8gF, AgF and BgF may be AgF=0.6414624845 and BgF=xe2x88x921.617829137xc3x9710xe2x88x923.
To satisfy ordinary achromatic conditions, on the other hand, it is generally required to satisfy the following Abbe number conditions (c) and (d) at the same time.
40 less than (xcexdd)pxe2x80x83xe2x80x83(c)
(xcexdd)n less than 30xe2x80x83xe2x80x83(d)
Here (xcexdd)p is the Abbe number of the positive lens element in the lens group A and (xcexdd)n is the Abbe number of the negative lens element in the lens group A.
Any failure in satisfying these conditions causes both longitudinal chromatic aberration and chromatic aberration of magnification to remain under-corrected, although the secondary spectra may be slackened.
In practice, special, if expensive, vitreous materials capable of satisfyin conditions (a) and (c) for a positive lens are available; however, there is no material that satisfies conditions (b) and (d) for a negative lens at the same time. For the negative lens, there is no option but to incorporate at least two negative lens elements in the lens group A in such a way that one element satisfies condition (b) and another satisfies condition (d). In consideration of the fact that the lens group A has positive refracting index, on the other hand, it is desired to set up the lens group A using positive lens elements equal or more in number to or than the negative lens elements, i.e., using two or more positive lens elements and two or more negative lens elements. The positive lens group A subsequent to the negative lens group is likely to be sensitive to decentration, and so it is preferable to cement together the positive lens element and the negative lens element. It is thus desired that the lens group A be composed of two doublet components C1 and C2, each consisting of a positive lens element and a negative lens element.
To attain another object of the invention, i.e., a zoom lens that has a short length and high resolving power while ensuring a high zoom ratio, the doublet component C2 should preferably have a meniscus form concave on its image side.
Furthermore, that doublet component C2 should preferably satisfy the following condition (e):
1.6 less than (RC21+RC22)/(RC21xe2x88x92RC22) less than 8.0xe2x80x83xe2x80x83(e)
where RC21 is the axial radius of curvature of the object side-surface of the doublet component located on the image side of the positive lens group, and RC22 is the axial radius of curvature of the image side-surface of the doublet component located on the image side of the positive lens group.
As the lower limit of 1.6 to condition (e) is not reached, it is difficult to make the length of the optical system short, and as the upper limit of 8.0 is exceeded, it is difficult to make correction for spherical aberrations and coma.
More preferably,
1.8 less than (RC21+RC22)/(RC21xe2x88x92RC22) less than 7.0xe2x80x83xe2x80x83(e)xe2x80x2
Most preferably,
2.0 less than (RC21+RC22)/(RC21xe2x88x92RC22) less than 6.0xe2x80x83xe2x80x83(e)xe2x80x3
It is acceptable that only the upper or lower limit to condition (e) is changed to the upper or lower limit to condition (e)xe2x80x2 or (e)xe2x80x3.
From another point of view, i.e., to shorten the length of the optical system, it is preferable to allow the doublet component C2 to have negative refracting power.
As mentioned above, the lens group A is constructed in such a way as to have therein two doublet components C1 and C2. To slack chromatic aberrations due to secondary spectra using the lens group A, it is preferable to satisfy the following condition (f). That is, it is preferable to satisfy the following medium condition (f) with respect to all the positive lens elements included in the lens group A.
xe2x80x830.000 less than AVE[(xcex94xcex8gF)pi] less than 0.080xe2x80x83xe2x80x83(f)
Here AVE[(xcex94xcex8gF)pi] is the arithmetic mean of the anomalous dispersibilities of all positive lens elements included in the positive lens group.
Suppose now that the positive lens group A does not include any positive lens with the exception of the positive lens elements included in the two doublet components C1 and C2. Then, AVE[(xcex94xcex8gF)pi] in condition (f) becomes
AVE[(xcex94xcex8gF)pi]=[(xcex94xcex8gF)p1+(xcex94xcex8gF)p2]/2
Here (xcex94xcex8gF)p1 is the anomalous dispersibility of the object side-positive lens element and (xcex94xcex8gF)p2 is the anomalous dispersibility of the image side-positive lens element.
Falling short of the lower limit of 0.0000 to condition (f) is not preferable because chromatic aberrations due to secondary spectra remain under-corrected and the chromatic blurring of an image becomes noticeable. A medium exceeding the upper limit of 0.080 does not occur in nature.
More preferably,
0.003 less than AVE[(xcex94xcex8gF)pi] less than 0.050xe2x80x83xe2x80x83(f)xe2x80x2
Most preferably,
0.006 less than AVE[(xcex94xcex8gF)pi] less than 0.020xe2x80x83xe2x80x83(f)xe2x80x3
It is acceptable that only the upper or lower limit to condition (f) is changed to the upper or lower limit to condition (f)xe2x80x2 or (f)xe2x80x3.
Alternatively, it is acceptable that the following conditions (1) and (2) for each lens medium inclusive of that of the negative lens element in the lens group A are separately or concurrently satisfied.                               xe2x80x83                ⁢                  0.000           less than                       AVE            ⁡                          [                                                                    (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        gF                                                              )                                    ⁢                  C1p                                ,                                                      (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        gF                                                              )                                    ⁢                  C2p                                            ]                                 less than           0.080                                    (        1        )                                          -          0.030                 less than                   AVE          ⁡                      [                                                            (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      gF                                                        )                                ⁢                C1n                            ,                                                (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      gF                                                        )                                ⁢                C2n                                      ]                           less than         0.003                            (        2        )            
Here
(xcex94xcex8gF)C1p is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C1n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C2p is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group,
(xcex94xcex8gF)C2n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group,
AVE[(xcex94xcex8gF)C1p , (xcex94xcex8gF)C2p ] is the arithmetic mean of (xcex94xcex8gF)C1p and (xcex94xcex8gF)C2p , and
AVE[(xcex94xcex8gF)C1n , (xcex94xcex8gF)C2n ] is the arithmetic mean of (xcex94xcex8gF)C1n and (xcex94xcex8gF)C2n. 
Any deviation from the lower limit of 0.000 and the upper limit of 0.003 to condition (1) is not preferable because chromatic aberrations due to secondary spectra remain and the chromatic blurring of an image becomes noticeable. A medium exceeding the upper limit of 0.080 to condition (1) or falling short of the lower limit of xe2x88x920.030 to condition (2) does not occur in nature.
It is more preferably to satisfy one or both of the following conditions (1)xe2x80x2 and (2)xe2x80x2:                               xe2x80x83                ⁢                  0.003           less than                       AVE            ⁡                          [                                                                    (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        gF                                                              )                                    ⁢                  C1p                                ,                                                      (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        gF                                                              )                                    ⁢                  C2p                                            ]                                 less than           0.050                                              (          1          )                ⁢        xe2x80x2                                          -          0.020                 less than                   AVE          ⁡                      [                                                            (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      gF                                                        )                                ⁢                C1n                            ,                                                (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      gF                                                        )                                ⁢                C2n                                      ]                           less than         0.000                                      (          2          )                ⁢        xe2x80x2            
It is even more preferable to satisfy either one of the following conditions (1)xe2x80x3 and (2)xe2x80x3, and it is most preferable to satisfy both the following conditions (1)xe2x80x3 and (2)xe2x80x3.                               xe2x80x83                ⁢                  0.006           less than                       AVE            ⁡                          [                                                                    (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        gF                                                              )                                    ⁢                  C1p                                ,                                                      (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        gF                                                              )                                    ⁢                  C2p                                            ]                                 less than           0.020                                              (          1          )                ⁢        xe2x80x3                                          -          0.010                 less than                   AVE          ⁡                      [                                                            (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      gF                                                        )                                ⁢                C1n                            ,                                                (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      gF                                                        )                                ⁢                C2n                                      ]                           less than                   -          0.003                                              (          2          )                ⁢        xe2x80x3            
It is acceptable that only the upper or lower limit to conditions (1) and (2) are changed to the upper or lower limit to conditions (1)xe2x80x2 and (2)xe2x80x2 or (1)xe2x80x3 and (2)xe2x80x3.
In the invention, there are additionally provided ordinary dichroic achromatic conditions (g) and (h).
50 less than AVE[(xcexdd)C1p, (xcexdd)C2p]xe2x80x83xe2x80x83(g)
AVE[(xcexdd)C1n, (xcexdd)C2n] less than 50xe2x80x83xe2x80x83(h)
Here
(xcexdd)C1p is the Abbe number on a d-line basis of a medium forming the positive lens element in the doublet component of the two doublet components, which component is located on the object side of said positive lens group,
(xcexdd)C2p is the Abbe number on a d-line basis of a medium forming the positive lens element in the doublet component of the two doublet components, which component is located on the image side of said positive lens group,
(xcexdd)C1n is the Abbe number on a d-line basis of a medium forming the negative lens element in the doublet component of the two doublet components, which component is located on the object side of said positive lens group,
(xcexdd)C2n is the Abbe number on a d-line basis of a medium forming the negative lens element in the doublet component of the two doublet components, which component is located on the image side of said positive lens group,
AVE[(xcexdd)C1p , (xcexdd)C2p ] is the arithmetic mean of (xcexdd)C1p and (xcexdd)C2p , and
AVE[(xcexdd)C1n , (xcexdd)C2n ] is the arithmetic mean of (xcexdd)C1n and (xcexdd)C2n .
Regarding condition (g), it is acceptable to set 85 as the upper limit. A lens material exceeding that upper limit costs much.
Regarding condition (h), it is acceptable to set 25 as the lower limit. A lens material below that lower limit again costs much.
It is more preferable to satisfy one or both of the following conditions (g)xe2x80x2 and (h)xe2x80x2.
55 less than AVE[(xcexdd)C1p, (xcexdd)C2p]xe2x80x83xe2x80x83(g)xe2x80x2
AVE[(xcexdd)C1n, (xcexdd)C2n] less than 45xe2x80x83xe2x80x83(h)xe2x80x2
It is even more preferable to satisfy one of the following conditions (g)xe2x80x3 and (h)xe2x80x3, and it is most preferable to satisfy both the following conditions (g)xe2x80x3 and (h)xe2x80x3.
60 less than AVE[(xcexdd)C1p, (xcexdd)C2p]xe2x80x83xe2x80x83(g)xe2x80x3
AVE[(xcexdd)C1n, (xcexdd)C2n] less than 40xe2x80x83xe2x80x83(h)xe2x80x3
It is acceptable that only the upper or lower limits to conditions (g) and (h) are changed to the upper or lower limits to conditions (g)xe2x80x2 and (h)xe2x80x2 or conditions (g)xe2x80x3 and (h)xe2x80x3.
For each lens medium in the lens group A, it is alternatively preferable to satisfy at least one of the following conditions (3-1) and (3-2) and at least one of the following conditions (4-1) and (4-2).                               xe2x80x83                ⁢                  0.0000           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C1p                     less than           0.1000                                    (                  3          ⁢                      -                    ⁢          1                )                                          xe2x80x83                ⁢                  0.0000           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C2p                     less than           0.1000                                    (                  3          ⁢                      -                    ⁢          2                )                                          -          0.0300                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C1n                 less than         0.0000                            (                  4          ⁢                      -                    ⁢          1                )                                          -          0.0300                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C2n                 less than         0.0000                            (                  4          ⁢                      -                    ⁢          2                )            
where
(xcex94xcex8gF)C1p is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C1n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(xcex94xcex8gF)C2p is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group, and
(xcex94xcex8gF)C2n is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the image side of said positive lens group.
Any deviation from the lower limit of 0.0000 to condition (3-1) or (3-2) or the upper limit of 0.0000 to condition (4-1) or (4-2) is not preferable because chromatic aberrations due to secondary spectra remain and the chromatic blurring of an image becomes noticeable. A medium exceeding the upper limit of 0.1000 to condition (3-1) or (3-2) or falling short of the lower limit of xe2x88x920.0300 to condition (4-1) or (4-2) does not occur in nature.
It is more preferable to satisfy at least one or all of the following conditions (3-1)xe2x80x2, (3-2)xe2x80x2, (4-1)xe2x80x2 and (4-2)xe2x80x2.                               xe2x80x83                ⁢                  0.0050           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C1p                     less than           0.0800                                              (                      3            ⁢                          -                        ⁢            1                    )                ⁢        xe2x80x2                                          xe2x80x83                ⁢                  0.0050           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C2p                     less than           0.0800                                              (                      3            ⁢                          -                        ⁢            2                    )                ⁢        xe2x80x2                                          -          0.0250                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C1n                 less than                   -          0.0030                                              (                      4            ⁢                          -                        ⁢            1                    )                ⁢        xe2x80x2                                          -          0.0250                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C2n                 less than                   -          0.0030                                              (                      4            ⁢                          -                        ⁢            2                    )                ⁢        xe2x80x2            
It is even more preferable to satisfy at least one of the following conditions (3-1)xe2x80x3, (3-2)xe2x80x3, (4-1)xe2x80x3 and (4-2)xe2x80x3, and it is most preferable to satisfy all of the following conditions (3-1)xe2x80x3, (3-2)xe2x80x3, (4-1)xe2x80x3 and (4-2)xe2x80x3.                               xe2x80x83                ⁢                  0.0100           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C1p                     less than           0.0600                                              (                      3            ⁢                          -                        ⁢            1                    )                ⁢        xe2x80x3                                          xe2x80x83                ⁢                  0.0100           less than                                     (                              Δ                ⁢                                  xe2x80x83                                ⁢                                  θ                  gF                                            )                        ⁢            C2p                     less than           0.0600                                              (                      3            ⁢                          -                        ⁢            2                    )                ⁢        xe2x80x3                                          -          0.0200                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C1n                 less than                   -          0.0060                                              (                      4            ⁢                          -                        ⁢            1                    )                ⁢        xe2x80x3                                          -          0.0200                 less than                               (                          Δ              ⁢                              xe2x80x83                            ⁢                              θ                gF                                      )                    ⁢          C2n                 less than                   -          0.0060                                              (                      4            ⁢                          -                        ⁢            2                    )                ⁢        xe2x80x3            
It is acceptable that only the upper or lower limits to conditions (3-1), (3-2), (4-1) and (4-2) are changed to the upper or lower limits to conditions (3-1)xe2x80x2, (3-2)xe2x80x2, (4-1)xe2x80x2 and (4-2)xe2x80x2 or conditions (3-1)xe2x80x3, (3-2)xe2x80x3, (4-1)xe2x80x3 and (4-2)xe2x80x3.
In the invention, there are additionally provided ordinary dichroic achromatic conditions (i-1), (i-2), (j-1) and (j-2). It is then desired to satisfy at least one of the following conditions (i-1) and (i-2), and at least one of the following conditions (j-1) and (j-2):
60 less than (xcexdd)C1pxe2x80x83xe2x80x83(i-1)
60 less than (xcexdd)C2pxe2x80x83xe2x80x83(i-2)
(xcexdd)C1n less than 30xe2x80x83xe2x80x83(j-1)
xe2x80x83(xcexdd)C2n less than 30xe2x80x83xe2x80x83(j-2)
Here
(xcexdd)C1p is the Abbe number on a d-line basis of a medium forming the positive lens element in the doublet component of the two doublet components, which component is located on the object side of said positive lens group,
(xcexdd)C2p is the Abbe number on a d-line basis of a medium forming the positive lens element in the doublet component of the two doublet components, which component is located on the image side of said positive lens group,
(xcexdd)C1n is the Abbe number on a d-line basis of a medium forming the negative lens element in the doublet component of the two doublet components, which component is located on the object side of said positive lens group, and
(xcexdd)C2n is the Abbe number on a d-line basis of a medium forming the negative lens element in the doublet component of the two doublet components, which component is located on the image side of said positive lens group,
Regarding conditions (i-1) and (i-2), it is acceptable to set 85 as the upper limit. A lens material exceeding that upper limit costs much. Regarding conditions (j-1) and (j-2), it is acceptable to set 25 as the lower limit. A lens material below that lower limit again costs much.
Now that the medium possessing anomalous dispersion is introduced in the optical system for correction of secondary spectra, it is important to enhance the effect of that medium.
The introduction of the medium into the positive lens element, to which a medium having especially high anomalous dispersibility is easily applied, is effective. The stronger the power of the positive lens element, the greater that effect becomes.
Thus, it is desired that any of the positive lens elements included in the positive lens group A satisfy the following condition (k).
0.00 less than "PHgr"pxc2x7(xcex94xcex8gF)pxc2x7L less than 0.1xe2x80x83xe2x80x83(k)
Here "PHgr"p is the refracting power in air of any positive lens element included in the positive lens group, (xcex94xcex8gF)p is the anomalous dispersibility of a medium forming the positive lens element, and L is the diagonal length in mm of an effective image pickup area of the image pickup device. It is here noted that the image pickup device is used on such an assumption as to include an angle of view of 55xc2x0 or greater at the wide-angle end of the optical system.
As the lower limit of 0.000 to condition (k) is not reached, it is impossible to make full correction of chromatic aberrations due to secondary spectra. Even when a medium occurring in nature and exceeding the upper limit of 0.1 is used, the power of the positive lens element becomes too strong, often giving rise to chromatic aberrations of spherical aberrations.
More preferably,
0.002 less than "PHgr"pxc2x7(xcex94xcex8gF)pxc2x7L less than 0.07xe2x80x83xe2x80x83(k)xe2x80x2
Most preferably,
0.004 less than "PHgr"pxc2x7(xcex94xcex8gF)pxc2x7L less than 0.05xe2x80x83xe2x80x83(k)xe2x80x3
It is acceptable that only the upper or lower limit to condition (k) is changed to the upper or lower limit to condition (k)xe2x80x2 or (k)xe2x80x3.
For any of the negative lens elements included in the positive lens group A, on the other hand, it is desired to satisfy the following condition (l):
0.000 less than "PHgr"nxc2x7(xcex94xcex8gF)nxc2x7L less than 0.03xe2x80x83xe2x80x83(l)
Here "PHgr"n is the refracting power in air of any of the negative lens elements included in the positive lens group, (xcex94xcex8gF)n is the anomalous dispersibility of a medium forming the negative lens element, and L is the diagonal length in mm of an effective image pickup area of the image pickup device. It is here noted that the image pickup device is used on such an assumption as to include an angle of view of 55xc2x0 or greater at the wide-angle end of the optical system.
As the upper limit of 0.03 to condition (l) is exceeded, it is impossible to make full correction of chromatic aberrations due to secondary spectra. Even when a medium occurring in nature and falling short of the lower limit of 0.000 is used, the power of the negative lens element becomes too strong, often giving rise to chromatic aberrations of spherical aberrations.
More preferably,
0.001 less than "PHgr"nxc2x7(xcex94xcex8gF)nxc2x7L less than 0.02xe2x80x83xe2x80x83(l)xe2x80x2
Most preferably,
0.002 less than "PHgr"nxc2x7(xcex94xcex8gF)nxc2x7L less than 0.01xe2x80x83xe2x80x83(l)xe2x80x3
It is acceptable that only the upper or lower limit to condition (1) is changed to the upper or lower limit to condition (1)xe2x80x2 or (1)xe2x80x3.
To reduce chromatic blurring, it is important just only to make correction of higher-order longitudinal chromatic aberrations and chromatic aberrations of magnification but also to make correction for chromatic spherical aberration, chromatic coma, etc. Although the addition of power to each lens element having effective anomalous dispersibility is favorable for correction of secondary spectra, yet it often causes chromatic spherical aberration and chromatic coma to become worse. This is particularly true for a negative lens element having excessive power.
It is thus preferable to satisfy the following condition (m).
xe2x88x922.8 less than ("PHgr"C1n+"PHgr"C2n)xc2x7L less than xe2x88x920.6xe2x80x83xe2x80x83(m)
Here
"PHgr"C1n is the refracting power in air of the negative lens element in the doublet component of the two doublet components, which component is located on the object side of the positive lens group,
"PHgr"C2n is the refracting power in air of the negative lens element in the doublet component of the two doublet components, which component is located on the image side of the positive lens group, and
L is the diagonal length in mm of an effective image pickup area of the image pickup device.
Falling short of the lower limit of xe2x88x922.8 to condition (m) makes chromatic spherical aberration and chromatic coma likely to occur, and exceeding the upper limit of xe2x88x920.6 makes correction of chromatic aberrations generally insufficient.
More preferably,
xe2x88x922.4 less than ("PHgr"C1n+"PHgr"C2n)xc2x7L less than xe2x88x920.8xe2x80x83xe2x80x83(m)xe2x80x2
Most preferably,
xe2x88x922.0 less than ("PHgr"C1n+"PHgr"C2n)xc2x7L less than xe2x88x921.0xe2x80x83xe2x80x83(m)xe2x80x3
It is acceptable that only the upper or lower limit to condition (m) is changed to the upper or lower limit to condition (m)xe2x80x2 or (m)xe2x80x3.
The Abbe number of a medium having anomalous dispersibility effective for a negative lens tends to become too large for the negative lens, and hence that medium tends to have power upon dichroic achromatization.
It is thus desired to satisfy the following conditions (o) and (p) separately or simultaneously.
xe2x80x83xe2x88x9220 less than (xcexdd)C1pxe2x88x92(xcexdd)C1n less than 50xe2x80x83xe2x80x83(o)
20 less than (xcexdd)C2pxe2x88x92(xcexdd)C2n less than 70xe2x80x83xe2x80x83(p)
Here
(xcexdd)C1p is the Abbe number on a d-line basis of a medium forming the positive lens element in the doublet component of the two doublet components, which component is located on the object side of said positive lens group,
(xcexdd)C2p is the Abbe number on a d-line basis of a medium forming the positive lens element in the doublet component of the two doublet components, which component is located on the image side of said positive lens group,
(xcexdd)C1n is the Abbe number on a d-line basis of a medium forming the negative lens element in the doublet component of the two doublet components, which component is located on the object side of said positive lens group, and
(xcexdd)C2n is the Abbe number on a d-line basis of a medium forming the negative lens element in the doublet component of the two doublet components, which component is located on the image side of said positive lens group,
By the way, when the lens group A is composed of a positive lens component and a negative lens component, it is easy to make well-balanced correction for chromatic aberration of magnification by providing a suitable air separation between both lens components on the optical axis. That is,
xe2x80x830.01 less than dA/L less than 0.06xe2x80x83xe2x80x83(q)
where dA is the air separation on the optical axis of the zoom lens between two doublet components in the positive lens group, and L is the diagonal length in mm of an effective image pickup area of the image pickup device.
As the lower limit of 0.01 to condition (q) is not reached, it is difficult to keep correction of chromatic aberration of magnification in balance all over the zooming zone, and the upper limit of 0.06 is exceeded, the sensitivity of both components to relative decentration tends to become high.
More preferably,
0.012 less than dA/L less than 0.05xe2x80x83xe2x80x83(q)xe2x80x2
Most preferably,
0.014 less than dA/L less than 0.04xe2x80x83xe2x80x83(q)xe2x80x3
It is acceptable to change only the upper or lower limit to condition (q) to the upper or lower limit to condition (q)xe2x80x2 or (q)xe2x80x3.
If the cementing surface in the doublet component of the two doublet components in the positive lens group, which component is located on the image side thereof, satisfies the following condition (5), then chromatic aberrations can be well reduced.
xe2x88x920.4 less than L/RC2C less than 1.2xe2x80x83xe2x80x83(5)
Here RC2C is the axial radius of curvature of the cementing surface in the doublet component of the two doublet components in the positive lens group, which component is located on the image side thereof, and L is the diagonal length in mm of an effective image pickup area of the image pickup device.
Falling short of the lower limit of xe2x88x920.4 to this condition is not preferred because longitudinal aberrations of short wavelength rays passing through the rim of the lens system tend to assume a large positive value and so chromatic blurring tends to occur at the edge site of a subject with a large brightness difference. As the upper limit of 1.2 is exceeded, both longitudinal chromatic aberration and chromatic aberration of magnification are likely to remain under-corrected. However, it is understood that when the zoom lens of the invention is used with an image pickup device in which the diagonal length of its effective image pickup area is represented by L, images can be picked up at an angle of view of 55xc2x0 or greater.
More preferably,
xe2x88x920.2 less than L/RC2C less than 0.9xe2x80x83xe2x80x83(5)xe2x80x2
Most preferably,
0 less than L/RC2C less than 0.6xe2x80x83xe2x80x83(5)xe2x80x3
When the lens group (the first lens group) located nearest to the object side of the lens system is a positive lens group, it is preferable to satisfy the following condition (r) regarding the amount of zooming movement of the lens group A, because it is easy to reduce fluctuations of coma and astigmatism with zooming.
xe2x80x83xe2x88x920.05 less than mST/mWS less than 0.15xe2x80x83xe2x80x83(r)
Here, on condition that the direction of movement of the positive lens group is positive on the image side, mWS is the amount of movement of the positive lens group from the wide-angle end to an intermediate focal length state, and mST is the amount of movement of the positive lens group from the intermediate focal length state to the telephoto end. The xe2x80x9cintermediate focal length statexe2x80x9d used herein is understood to refer to a state where the intermediate focal length can be defined by the geometric mean of the focal length at the wide-angle end and the focal length at the telephoto end. It is here noted that the lens group A does not move along the optical axis of the lens system during focusing.
Generally in a zoom lens of the type comprising, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power and at least one subsequent lens group having positive refracting power, the amount of zooming movement of the second lens group tends to become large especially on the wide-angle side upon zooming from the wide-angle end to the telephoto end. It is thus preferable to use most of the overall amount of movement of the lens group A (the positive lens group) for multiplication on the wide-angle side, because fluctuations of aberrations in association with fluctuations of meridional field curvature are reduced.
As the upper limit of 0.15 to condition (r) is exceeded, fluctuations of meridional field curvature on the wide-angle side tend to become large, and as the lower limit of xe2x88x920.05 is not reached, the effect of the lens group A on multiplication is not expectable.
More preferably,
xe2x88x920.03 less than mST/mWS less than 0.12xe2x80x83xe2x80x83(r)xe2x80x2
Most preferably,
xe2x88x920.01 less than mST/mWS less than 0.1xe2x80x83xe2x80x83(r)xe2x80x3
It is acceptable to change only the upper or lower limit to condition (r) to the upper or lower limit to condition (r)xe2x80x2 or (r)xe2x80x3.
When the group located nearest to the object side (the first lens group) has positive refracting power and the second lens group has negative refracting power, the zoom lens of the invention is characterized by having a high zoom ratio. To reduce and stabilize fluctuations of meridional field curvature in particular all over the zooming zone, the construction of the second lens group is of importance. Regarding the negative lens element located nearest to the object side of the second lens group, which negative lens element has the highest off-axis ray within the second lens group and has strong power, it is thus preferable to satisfy the following condition (s).
1.83 less than n21 less than 2.01xe2x80x83xe2x80x83(s)
Here n21 is the d-line refractive index of a medium forming the negative lens element.
As the lower limit of 1.83 to condition (s) is not reached, it is difficult to reduce and stabilize fluctuations of meridional field curvature, and a medium exceeding the upper limit of 2.01 does not occur in nature.
More preferably,
1.84 less than n21 less than 2.01xe2x80x83xe2x80x83(s)xe2x80x2
Most preferably,
1.85 less than n21 less than 2.01xe2x80x83xe2x80x83(s)xe2x80x3
It is acceptable to change only the upper or lower limit to condition (s) to the upper or lower limit to condition (s)xe2x80x2 or (s)xe2x80x3.
Alternatively, the second lens group should preferably include a doublet component consisting of, in order from its object side, a negative lens element and a positive lens element, and satisfy the following condition (t).
0.25 less than ncpxe2x88x92nen less than 0.55xe2x80x83xe2x80x83(t)
Here ncp and nen are the d-line refractive indices of media forming the positive lens element and the negative lens element in any of doublet components included in the second lens group, respectively.
As the lower limit of 0.25 to condition (t) is not reached, it is difficult to reduce and stabilize fluctuations of meridional field curvature. A medium exceeding the upper limit of 0.55 does not occur in nature.
More preferably,
xe2x80x830.26 less than ncpxe2x88x92nen less than 0.55xe2x80x83xe2x80x83(t)xe2x80x2
Most preferably,
0.27 less than ncpxe2x88x92nen less than 0.55xe2x80x83xe2x80x83(t)xe2x80x3
It is acceptable to change only the upper or lower limit to condition (t) to the upper or lower limit to condition (t)xe2x80x2 or (t)xe2x80x3.
By the way, in such a telephoto state that the diagonal angle of view all over the effective screen is below 10xc2x0, chromatic blurring due to the influence of secondary spectra occurs predominantly. It is preferable to use a vitreous material having anomalous dispersiblity for a lens having a particularly high axial ray in a telephoto state and large refracting power because that lens can be greatly corrected for chromatic blurring. It is thus preferable to satisfy the following condition (u) regarding at least one positive lens element in the first lens group.
0.004 less than (xcex94xcex8gF)* less than 0.1xe2x80x83xe2x80x83(u)
Here (xcex94xcex8gF)* is the anomalous dispersibility of any positive lens element in the first lens group.
Falling short of the lower limit of 0.004 to condition (u) is not preferred because chromatic aberrations (both longitudinal chromatic aberration and chromatic aberration of magnification) due to secondary spectra in the neighborhood of the telephoto end remain and so the chromatic blurring of an image becomes noticeable. A medium exceeding the upper limit of 0.1 does not occur in nature.
In addition, it is desired to satisfy the following condition (v) regarding another positive lens element in the first lens group.
0.000 less than (xcex94xcex8gF)** less than 0.1xe2x80x83xe2x80x83(v)
Here xcex94xcex8gF)** is the anomalous dispersibility of the positive lens element in the first lens group, which is different from the aforesaid any positive lens element.
As the lower limit of 0.000 to condition (v) is not reached, removal of chromatic aberrations (both longitudinal chromatic aberration and chromatic aberration of magnification) due to secondary spectra in the vicinity of the telephoto end becomes insufficient and so the chromatic blurring of an image remains. A medium exceeding the upper limit of 0.1 does not occur in nature.
It is particularly preferred to use the aforesaid medium for the lens element that has the greatest refracting power among the first lens group. Usually, the first lens group is often composed of three lens elements, i.e., a negative lens element, a positive lens element and a positive lens element, with the middle positive lens element having the greatest refracting power.
More preferably,
0.01 less than (xcex94xcex8gF)* less than 0.08xe2x80x83xe2x80x83(u)xe2x80x2
Most preferably,
xe2x80x830.02 less than (xcex94xcex8gF)* less than 0.06xe2x80x83xe2x80x83(u)xe2x80x3
Regarding condition (v), too, it is more preferable that
0.0020 less than (xcex94xcex8gF)** less than 0.08xe2x80x83xe2x80x83(v)xe2x80x2
Most preferably,
0.0040 less than (xcex94xcex8gF)** less than 0.06xe2x80x83xe2x80x83(v)xe2x80x3
It is acceptable that only the upper or lower limits to conditions (u) and (v) are changed to the upper or lower limits to conditions (u)xe2x80x2, (v)xe2x80x2 or (u)xe2x80x3, (v)xe2x80x3.
To reduce and stabilize fluctuations of meridional field curvature in particular all over the zooming zone, the lens group located nearest to the image side of the lens system is provided with a lens element both surfaces of which are formed of aspherc surfaces, and which satisfies the following condition (w). It is noted that for focusing, the lens group located nearest to the image side moves on the optical axis of the lens system.
xe2x88x920.05 less than (AspRRxe2x88x92AspRF)/L less than 0xe2x80x83xe2x80x83(w)
Here
AspRF is the amount of axial displacement of the object side-surface of the lens component having aspheric surfaces on both sides with respect to a spherical surface having an axial radius of curvature, as measured at a height of 0.4 L from the optical axis,
AspRR is the amount of axial displacement of the image side-surface of the lens component having aspheric surfaces on both sides with respect to a spherical surface having an axial radius of curvature, as measured at a height of 0.4 L from the optical axis, and
L is the diagonal length in mm of an effective image pickup area of the image pickup device. It is here noted that when that lens element moves toward the object side, the amount of displacement takes on a negative value. Referring to FIG. 26, the xe2x80x9camount of displacement of an aspheric surfacexe2x80x9d used herein is understood to mean the amount of displacement of that aspheric surface with respect to a spherical surface (reference surface) having a radius of curvature, r, on the optical axis of that aspheric surface, as measured at a height of 0.4 L from the optical axis, where L is the diagonal length of the effective image pickup area of the image pickup device.
As the upper limit of 0 to condition (w) is exceeded, it is difficult to reduce and stabilize fluctuations of meridional field curvature, and as the lower limit of xe2x88x920.05 is not reached, it is difficult to reduce and stabilize fluctuations of meridional field curvature upon focusing.
More preferably,
xe2x88x920.03 less than (AspRRxe2x88x92AspRF)/L less than 0xe2x80x83xe2x80x83(w)xe2x80x2
Most preferably,
xe2x88x920.01 less than (AspRRxe2x88x92AspRF)/L less than 0xe2x80x83xe2x80x83(w)xe2x80x3
It is acceptable that only the upper or lower limit to condition (w) is changed to the upper or lower limit to condition (w)xe2x80x2 or (w)xe2x80x3.
As explained so far, chromatic aberrations become a problem when it is required to make full use of the performance of a zoom lens having a high zoom ratio, a small F-number and a long length while used in combination with an image pickup device having much more pixels. The zoom lens having a high zoom ratio is herein represented by one that satisfies the following conditions (x) and (y) provided that the third and subsequent lens groups are collectively called the rear lens group.
1.2xe2x89xa6xe2x88x92xcex22Txe2x89xa610xe2x80x83xe2x80x83(x)
0.1xe2x89xa6xe2x88x92xcex2RTxe2x89xa60.6xe2x80x83xe2x80x83(y)
Here xcex22T is the magnification at the telephoto end of the negative lens group upon focused on an object point at infinity, and xcex2RT is the magnification at the telephoto end of all lens groups located on the image side with respect to the negative lens group upon focused on an object point at infinity.
Condition (x) provides a definition of the magnification at the telephoto end of the second lens group when a certain or higher zoom ratio is obtained. As the upper limit of 10 is exceeded, it is difficult to ensure a certain angle of view on the wide-angle end, and as the lower limit of 1.2 is not reached, the zooming effect tends to become slender relative to the amount of movement of the second lens group.
Condition (y) provides a definition of the magnification of the rear lens group at the telephoto end upon focused on an object point at infinity. In a digital camera using an image pickup device by far much reduced in size than that used with a 35-mm film size camera, the focal length of an optical system is very short. The principal point of the digital camera system is located at a position considerably near to the image side of the optical system. In other words, there is no option but to use an extremely asymmetric refracting power profile, leading readily to the occurrence of chromatic aberration of magnification. The present invention is applicable only to such digital cameras, i.e., to an optical system coming within the range defined by condition (y).
It is more preferable to satisfy the following conditions (x)xe2x80x2 and/or (y)xe2x80x3.
1.6xe2x89xa6xe2x88x92xcex22Txe2x89xa610xe2x80x83xe2x80x83(x)xe2x80x2
0.15xe2x89xa6xe2x88x92xcex2RTxe2x89xa60.45xe2x80x83xe2x80x83(y)xe2x80x2
It is even more preferable to satisfy either one of the following conditions (x)xe2x80x3 and (y)xe2x80x3, and it is most preferable to satisfy both the following conditions (x)xe2x80x3 and (y)xe2x80x3.
2.0xe2x89xa6xe2x88x92xcex22Txe2x89xa610xe2x80x83xe2x80x83(x)xe2x80x3
0.2xe2x89xa6xe2x88x92xcex2RTxe2x89xa60.4xe2x80x83xe2x80x83(y)xe2x80x3
It is acceptable that only the upper or lower limits to conditions (x) and (y) are changed to the upper or lower limits to conditions (x)xe2x80x2, (y)xe2x80x2 or (x)xe2x80x3, (y)xe2x80x3.
The xe2x80x9cimage pickup device having much more pixelsxe2x80x9d is understood to refer to one that satisfies the following condition (z).
2xc3x9710xe2x88x922xe2x89xa6sxc2x7p/Lxe2x89xa64xc3x9710xe2x88x922xe2x80x83xe2x80x83(z)
Here
p/L satisfies p/Lxe2x89xa64.5xc3x9710xe2x88x924,
s is the actual length in mm of the lens system from the surface located nearest to the object side of the lens system to the image pickup surface as measured at the wide-angle end of the lens system upon focused on an object point at infinity,
p is a horizontal pixel pitch in mm, and
L is the diagonal length in mm of an effective image pickup area of the image pickup device.
Condition (z) provides a definition of the optical length and the number of pixels at the wide-angle end upon focused on an object point at infinity. The image-formation capability commensurate with a multiplicity of pixels may be ensured by increasing the size of the optical system; however, it does not make sense for a still camera. Exceeding the upper limit of 4xc3x9710xe2x88x922 to condition (z) renders it impossible to reduce the size of the camera. As the lower limit of 2xc3x9710xe2x88x922 is not reached, it is difficult to make correction for aberrations inclusive of chromatic blurring and, hence, to ensure image quality enough for the still camera, although size reductions may be achievable.
According to the invention, it is possible to provide an electronic image pickup apparatus comprising a zoom lens composed of, in order from its object side, a first lens group having positive refracting power and comprising a negative lens element, a second lens group having negative refracting power and comprising a positive lens element and a rear lens group comprising at least one lens group that is movable for zooming, an optical element formed only of a substantially planar surface and an electronic image pickup device, wherein the optical element formed only of a substantially planar surface is an optical filter that simultaneously satisfies transmission properties defined by the following conditions (xcex1) and (xcex2). For the optical system, this arrangement is essentially required to obtain a high zoom ratio.
The first lens group, because of being a converging system, has an action to lower axial light rays and an action to provide a finite object point with respect to the second lens group, so that the second lens group can be set up as a diverging system having strong refracting power thereby obtaining a great zooming action in a reduced amount of movement. In addition, the second lens group has an enhanced ability to correct aberrations while it is less susceptible to fluctuations of aberrations with zooming, and is easily applied to an image pickup device including as many pixels as 2,000,000 or greater.
As already described, however, the gravest problem with such a high-zoom-ratio zoom lens is chromatic aberrations due to secondary spectra, which cannot be corrected with an ordinary vitreous material. If a vitreous material having specific dispersion properties (anomalous dispersibility) is used as in the invention, then those chromatic aberrations may be corrected to some, if not sufficient, degrees. Still, with decreasing size, for instance, the refracting powers of the first, second and rear lens groups become too strong to make correction for longitudinal chromatic aberration and chromatic aberration of magnification on the telephoto side in particular and chromatic aberration of magnification on the wide-angle side, and chromatic blurring becomes unacceptable. Referring to an optical medium having general properties, these chromatic aberrations start increasing at a wavelength lower than 450 nm in particular, leading to remarkably purple blurring at 400 nm to 430 nm, to which an image pickup device shows high spectral sensitivity. On the other hand, the (bright-field) specific visual sensitivity of the human eyes is slight at 430 nm or lower.
Accordingly, if an absorber or reflector is inserted on the optical path, which is designed such that the ratio of the transmittance (xcfx84400) at 400 nm wavelength to that (xcfx84550) at 550 nm wavelength is less than 0.08 and the ratio of the transmittance (xcfx84440) at 440 nm wavelength to that (xcfx84550) at 550 nm wavelength is greater than 0.4, as defined by the following conditions (xcex1) and (xcex2), it is then possible to considerably reduce noises such as color blurring while the wavelength area necessary for color reproduction (satisfactory color reproduction) is kept intact.
Thus, the coating that can satisfy the transmittance properties of conditions (xcex1) and (xcex2) at the same time can be applied on the interior, or on the image side, of a lens system of the design and construction vulnerable to chromatic blurring, so that the absolute amount of chromatic blurring can largely be reduced without detrimental to color reproducibility.
xcfx84400/xcfx84550xe2x89xa60.08xe2x80x83xe2x80x83(xcex1)
xcfx84440/xcfx84550xe2x89xa70.4xe2x80x83xe2x80x83(xcex2)
Here xcfx84400, xcfx84440, and xcfx84550 is the transmittance at 400 nm, 440 nm, and 550 nm, respectively, of an optical element composed of a substantially planar surface.
More preferably, the following conditions (xcex1)xe2x80x2 and/or (xcex2)xe2x80x2 should be satisfied.
xcfx84400/xcfx84550xe2x89xa60.06xe2x80x83xe2x80x83(xcex1)xe2x80x2
xcfx84440/xcfx84550xe2x89xa70.5xe2x80x83xe2x80x83(xcex2)xe2x80x2
Even more preferably, the following condition (xcex1)xe2x80x3 or (xcex2)xe2x80x3 should be satisfied.
xe2x80x83xcfx84400/xcfx84550xe2x89xa60.04xe2x80x83xe2x80x83(xcex1)xe2x80x3
xcfx84440/xcfx84550xe2x89xa70.6xe2x80x83xe2x80x83(xcex2)xe2x80x3
Most preferably, both the following condition (xcex1)xe2x80x3 and (xcex2)xe2x80x3 should be satisfied.
xcfx84400/xcfx84550xe2x89xa60.04xe2x80x83xe2x80x83(xcex1)xe2x80x3
xcfx84440/xcfx84550xe2x89xa70.6xe2x80x83xe2x80x83(xcex2)xe2x80x3
It is acceptable that only the upper or lower limit to conditions (xcex1) and (xcex2) are changed to the upper or lower limits to conditions (xcex1)xe2x80x2, (xcex2)xe2x80x2 or (xcex1)xe2x80x3, (xcex2)xe2x80x3.
A problem with a plate form of medium having planar surfaces on both sides or a planar surface coated with a reflector used as the aforesaid absorber is that when the absorber is located at the entrance surface of an optical system almost vertically to the optical path, light of a high-brightness image point once formed on an image pickup device is reentered in a substantially afocal form onto the planar surface and an inverted image point is again formed as a ghost, causing considerable damage to image quality. This is because the image pickup device behaves as a specular reflector. When the absorber takes on a planar form, it must thus be located within an image pickup optical system. Preferably in this case, the absorber should be located on the surface of, or in the vicinity of, an optical low-pass filter. For the absorber having a curved surface like a lens, it is required that the angle of incidence and emergence of an axial marginal light ray at a reflecting surface and a transmitting surface be 1xc2x0 or greater with respect to the normal.
For a solid-state image pickup device having high sensitivity to an infrared range, a filter that absorbs infrared components is generally located in the neighborhood of the image pickup device. This absorber has a transmittance property of decreasing with a gentle gradient from 550 nm to 700 nm; however, that transmittance does not drop to zero at 700 nm. In the case of an image pickup device using a complementary colors mosaic filter, the transmittance of the filter with respect to magenta, blue and red regions is still low whereas the transmittance of the filter with respect to a green region is still high. For these and other reasons, a hue poor in color reproducibility appears. Thus, a hue of the visible range of 450 nm or lower, which is a main component of chromatic blurring that must originally be deep bluish purple, turns to reddish purple, and so the chromatic blurring becomes more unpleasant.
In the present invention, therefore, a coating that can satisfy the following conditions (xcex3) and (xcex4) at the same time is applied onto one plane (or the other plane when the coating according to the aforesaid conditions.is applied) of such an optical element comprising a substantially planar surface as mentioned above, which is located in the interior of, or on the image side of, a lens system of the design and construction vulnerable to chromatic blurring, so that the chromatic blurring hue can be returned back to less noticeable bluish purple that is the original hue.
xcfx84600/xcfx84550xe2x89xa70.8xe2x80x83xe2x80x83(xcex3)
xcfx84700/xcfx84550xe2x89xa60.08xe2x80x83xe2x80x83(xcex4)
Here xcfx84550, xcfx84600, and xcfx84700 is the transmittance at 550 nm, 600 nm, and 700 nm, respectively, of an optical element composed of a substantially planar surface.
It is more preferable to satisfy the following conditions (xcex3)xe2x80x2 and/or (xcex4)xe2x80x2.
xcfx84600/xcfx84550xe2x89xa70.85xe2x80x83xe2x80x83(xcex3)xe2x80x2
xcfx84700/xcfx84550xe2x89xa60.05xe2x80x83xe2x80x83(xcex4)xe2x80x2
It is even more preferable to satisfy the following conditions (xcex3)xe2x80x3 or (xcex4)xe2x80x3.
xcfx84600/xcfx84550xe2x89xa70.9xe2x80x83xe2x80x83(xcex3)xe2x80x3
xcfx84700/xcfx84550xe2x89xa60.03xe2x80x83xe2x80x83(xcex4)xe2x80x3
Most preferably, both conditions (xcex3)xe2x80x3 and (xcex4)xe2x80x3 should be satisfied.
xcfx84600/xcfx84550xe2x89xa70.9xe2x80x83xe2x80x83(xcex3)xe2x80x3
xcfx84700/xcfx84550xe2x89xa60.03xe2x80x83xe2x80x83(xcex4)xe2x80x3
It is acceptable that only the upper or lower limit to conditions (xcex3) and (xcex4) is changed to the upper or lower limit to conditions (xcex3)xe2x80x2, (xcex4)xe2x80x2 or (xcex3)xe2x80x3, (xcex4)xe2x80x3.
Further size reductions are achievable by setting up the aforesaid rear group with a plurality of lens groups that move separately on the optical axis during zooming. Preferably in this case, at least one of the plurality of lens groups is designed to move monotonously toward the object size upon zooming from the wide-angle end to the telephoto end.
While it is not always required to make correction for secondary spectral components, it is understood that if chromatic aberrations for each wavelength are placed in a well-balanced state as by reducing chromatic aberrations corresponding to 420 nm while chromatic aberrations corresponding to 540 nm and having high spectral sensitivity are slightly sacrificed, it is then possible to render chromatic blurring less noticeable to some extents. However, there are some limitations because this lead to deterioration in resolving power or MTF.
Chromatic aberration of magnification for each wavelength should preferably be corrected in such a way that when the optical system is focused at the wide-angle end on an object point at infinity,
g-line chromatic aberration of magnification with respect
to d-line less than 0
at an image height 0.7 time as large as an effective diagonal length, and
g-line chromatic aberration of magnification with respect to d-line less than h-line chromatic aberration of magnification
with respect to d-line
and that when the optical system is focused at the telephoto end on an object point at infinity,
h-line chromatic aberration of magnification with respect to d-line less than C-line chromatic aberration of magnification with respect to d-line less than g-line chromatic aberration of magnification with respect to d-line
at an image height 0.7 time as large as the effective diagonal length. Here C-line is 656.27 nm, d-line is 587.56 nm, g-line is 435.84 nm, and h-line is 404.66 nm.
To add to this, it is preferable to satisfy the relation:
g-line longitudinal chromatic aberration with respect to d-line less than C-line longitudinal chromatic aberration with respect to d-line
when the optical system is focused at the telephoto end on an object point at infinity.
Moreover, it is preferable to satisfy the following relations:
g-line longitudinal chromatic aberration with respect to d-line less than C-line longitudinal chromatic aberration with
respect to d-line
g-line longitudinal chromatic aberration with respect to d-line less than h-line longitudinal chromatic aberration with
respect to d-line
when the optical system is focused at the wide-angle end on an object point at infinity.
The optical system of the invention is now explained at great length. The zoom lens of the invention comprises, in order from its object side, a first lens group having positive refracting power and including a negative lens element, a second lens group having negative refracting power and including a positive lens element, a stop, and a third lens group having generally positive refracting power and including two doublet components C1 and C2, each of which is composed of, in order from its object side, a positive lens element and a negative lens element. Subsequent to the third lens group, the zoom lens may comprise a lens group or groups, and the third and subsequent lens groups are all independently movable for the purpose of reducing the length of the zoom lens and the diameter of the first lens group. Especially if the third lens group is moved in such a way that it is located nearer to the object side at the telephoto end rather than at the wide-angle end, the space for zooming movement of the second lens group is reduced and the entrance pupil is located at a shallow position, so that the diameter of the first lens group can be decreased. To make short the length of a partial system from the stop to the rear lens group, the third lens group is composed of two components, that is, in order from its object side, a positive lens component C1 and a negative lens component C2 having a concave surface on its image side. Having aspheric surfaces on both sides, the final lens element is movable on the optical axis for focusing.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.